JPH07101375B2 - Constant voltage power supply circuit - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to a constant voltage power supply circuit for digital circuits in which a power supply is divided into a backup power supply and a main power supply.

[Prior art]

Generally, in digital circuits that require a constant voltage power supply circuit,
For power supply voltage control, power supply control IC (TA7900S manufactured by Toshiba
Etc.) and the like to form a circuit configuration as shown in FIG. In FIG. 4, 1 is a power supply control IC, 2 is a transistor, 5 is a control power supply input line, 7 is a main power supply line, and 8 is a power supply voltage monitoring point. In such a configuration,
The power supply control IC 1 monitors the collector output voltage of the external transistor 2 and controls the base current of the transistor 2 to obtain a constant voltage. In this circuit, when a backup power supply that continuously supplies power is required, the constant voltage power supply must be separated into a main power supply and a backup power supply. The backup power supply is a circuit that charges a battery or the like when current is supplied from the constant voltage power supply, and temporarily supplies power from the battery when the power supply goes down for some reason. If power is supplied from the battery to the entire system when the power is down, current consumption will increase and a battery with a large capacity will be required. However, it is sufficient to be able to back up only a part such as a memory in which the contents do not change when the power is down. For this reason, a circuit configuration as shown in FIG. 5 has been considered. In FIG. 5, the same reference numerals as those in FIG. 4 indicate the same parts, but other than that, a backup power supply line 6, a diode 10, and a battery 11 are provided. In this way, in the circuit shown in FIG. 5, the part that needs to be backed up is separated from the entire system when the power supply is down by connecting the diode 10 to the backup power supply line 6 side, and the battery of the backup power supply is connected to the main power supply line 7 side. I am trying to prevent current from flowing in from 11. FIG. 6 shows another circuit configuration in which only a part such as a memory can be backed up when the power is turned off. The circuit of FIG. 6 is different from that of FIG. 5 in that instead of the diode 10, a backup IC for separating the main power supply and the backup power supply when the power supply is down (Fujitsu MB3780
Etc.) to control the power supply voltage.

[Problems to be Solved by the Invention]

When the diode is connected only to the backup power source as shown in FIG. 5, there is a problem that the potential of the backup power source becomes lower than that of the main power source by the forward voltage drop of the diode. Further, in a circuit that requires a backup power supply, in order to control the power supply voltage, the method as shown in FIG. 6 needs to add a backup IC separately, which increases the cost and increases the mounting area. There was a problem of being lost. In view of the above, the present invention can control the power supply voltage without separately adding a backup IC when the main power supply and the backup power supply are separated, and make the potentials of the main power supply and the backup power supply the same. It is an object of the present invention to provide a constant voltage power supply circuit capable of performing the above.

[Means for Solving the Problems]

When the diode is connected only to the backup power supply,
In order to solve the problem that the voltage of the backup power supply drops below the voltage of the main power supply, connect diodes to both the main power supply and the backup power supply, as shown in Fig. 1.
The voltage monitoring point by the power supply control IC is provided on the cathode side of the diode connected to the main power supply side. Further, the main power supply and the backup power supply have different current consumptions, and usually the main power supply consumes a large current. Therefore, diodes having different current capacities are used so that the forward voltage drops between the anode and the cathode become equal depending on the current consumption of both.

[Work]

By connecting diodes with different current capacities according to the current consumption of both the main and backup power supplies, the voltage drop across the diodes of the main power supply and backup power supply will be the same. Therefore, power control IC
If the power supply voltage monitoring point is provided for either the main power supply or the backup power supply, it is possible to control both power supply voltages. That is, by providing a power supply voltage monitoring point on the cathode side of the diode connected to the main power supply and controlling the voltage of the main current, the backup power supply can be kept at the same voltage.

【Example】

FIG. 1 is a circuit configuration diagram showing an embodiment of a constant voltage power supply circuit according to the present invention. In the circuit of FIG. 1, 3 and 4 are diodes, and those designated by other reference numerals are the same as those of FIG.
As the diodes 3 and 4, those having forward voltage-forward current characteristics such that the voltage drops are the same at the respective current consumptions of the main power supply and the backup power supply are selected and used. FIG. 3 is a forward voltage-forward current characteristic diagram of two diodes 3 and 4, in which a diode 4 having a large forward current for the same forward voltage drop is connected to the main power supply side and a diode 3 having a small forward current is connected. To the backup power supply side. Power control IC
Reference numeral 1 controls the voltage of both the main power supply and the backup power supply by providing the power supply monitoring point 8 on the cathode side of the diode 4. Of course, the power supply monitoring point 8 may be provided on the cathode side of the diode 3. FIG. 2 is a circuit configuration diagram showing another embodiment of the constant voltage power supply circuit according to the present invention. In the circuit of FIG. 2, diodes 12 and 13 having forward voltage-forward current characteristics such that the voltage drops are the same as in the embodiment of FIG. 1 are used.
In consideration of this voltage drop (usually 0.6 V), the output voltage from the constant voltage power supply 10 is increased in advance by the voltage drop.

【The invention's effect】

According to the present invention, in a circuit requiring a backup power supply, the main power supply and the backup power supply can be separated and the potential difference between the two power supplies can be eliminated. Here, even when the current consumption of the main power supply changes during operation, the use of a diode with a large current capacity reduces the ratio of forward voltage drop change to forward current change, as shown in the graph of FIG. The fluctuation of can be kept small. Moreover, since only two diodes are newly added, the mounting area is small, the cost is low, and the reliability is not impaired.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of a constant voltage power supply circuit according to the present invention, FIG. 2 is a circuit configuration diagram showing another embodiment of the constant voltage power supply circuit according to the present invention, and FIG. A voltage-forward current characteristic diagram, FIG. 4, FIG. 5, and FIG. 6 are circuit configuration diagrams of a conventional constant voltage power supply circuit. 1: Power control IC, 2: Transistor, 3,4,10,12,13: Diode, 5: Control power input line, 6: Backup power line,
7: Main power line, 8: Power voltage monitoring point, 9: Backup
IC, 11: Backup power supply battery.

Claims (3)

[Claims] 1. An anode side of two diodes having a characteristic that an equal forward voltage drop occurs between the anode and the cathode according to the current consumption of the power source, is connected in parallel to the output side of the constant voltage power source, and the cathode of each diode is connected. A constant voltage power supply circuit, wherein a side is connected to a main power supply line connected to a load and a back-up power supply line connected to a part of the load. 2. The constant voltage power supply circuit according to claim 1, wherein the constant voltage power supply is a control transistor.
A constant voltage power supply circuit comprising: a cathode of one of the two diodes as a voltage monitoring point, and a power supply control IC that controls the control transistor according to the voltage at the voltage monitoring point. 3. The constant voltage power supply circuit according to claim 1, wherein an output voltage value of the constant voltage power supply is set to a value higher by a voltage drop of the diode. circuit.